Wood consists of three major constituents: cellulose, hemicellulose and lignin. Cellulose and hemicellulose are already utilized in industry, in particular in the papermaking industry. Each year this use generates several million tonnes of lignin-rich byproducts, which are used as fuels of low calorific value for supplying heat and energy for the papermaking processes. In parallel, a minimal amount of lignin is isolated by direct extraction from plants (F. G. Calvo-Flores and J. A. Dobado, ChemSusChem. 2010, 3, pages 1227-1235).
Lignin is the most abundant substance in nature in terms of a source of aromatic groups and the greatest contributor of organic matter to the soil (S. Y. Lin, in Methods in Lignin Chemistry, Springer Series in Wood Science (Ed.: C. W. Dence), Springer, Berlin 1992). It results from the radical polymerization of three monomers called monolignols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which after polymerization by dehydrogenation with peroxidase give the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) residues respectively, as illustrated in FIG. 1 (R. Vanholme, K. Morreel, J. R. W. Boerjan, Curr. Opin. Plant Biol. 2008, 11, pages 278-285).
The complexity and the diversity of the structure of lignin largely depend on its origin. Taking plant taxonomy as a basis, it has been proposed that lignin from gymnosperms (called softwood, or “bois tendre” in French) has more G residues, than that from the angiosperms (called hardwood, or “bois dur” in French), which contains a mixture of residues G and S, and the lignin from herbaceous plants contains a mixture of the three aromatic residues H, G and S. A more rigorous classification technique was to adopt a chemical approach as a basis, in which the lignins are classified according to the abundance of the units G, H and S in the polymer. Four main groups of lignin have thus been identified: type G, type GS, type HGS and type HG (F. G. Calvo-Flores and J. A. Dobado, ChemSusChem. 2010, 3, pages 1227-1235).
Regardless of the type of lignin, this biopolymer is characterized by considerable chemical heterogeneity and consists of propyl-phenol units joined together by various types of C—O and C—C bonds of the aryl ether, aryl glycerol and β-aryl ether type. FIG. 2 shows the structure of lignin proposed by E. Adler, Wood Sci. Technol. 1977, 11, page 169.
Ether bonds represent about two thirds of the bonds. More specifically, the bonds of the β-O-4 and α-O-4 type, which form part of the alkaryl ethers, are the most abundant. Typically, lignin from angiosperms (hardwood, or bois dur in French) contains 60% of bonds of the β-O-4 type and 6-8% of the α-O-4 type, and lignin from gymnosperms (softwood or bois tendre in French) contains 46% of bonds of the β-O-4 type and 6-8% of the α-O-4 type. Although the proportion of these bonds varies considerably from one species to another, typical values taken from M. P. Pandey, C. S. Kim, Chem. Eng. Technol., 2011, 34, 29, are listed in the table in FIG. 3.
The chemical structures of the most abundant types of bonds in lignin are shown in FIG. 4.
Lignin represents the largest renewable reservoir of available aromatic compounds. Owing to its high aromatics content, lignin has great potential for functioning as an alternative to the nonrenewable fossil resources for producing aromatic chemicals with high added value, i.e. products whose transformation increases their commercial value considerably. As aromatic chemicals with high added value, we may mention, for example, 4-propylbenzene-1,2-diol (at 3700 $/kg) or 4-(3-hydroxypropyl)-1,2-benzenediol (at 3100 $/kg). Thus, upgrading of lignin involves its conversion to valuable, useful aromatic products via its depolymerization. However, owing to its amorphous, polymeric structure based on strong ether bonds, its depolymerization to produce usable molecules presents a challenge. Moreover, lignins are very varied structurally and, depending on the plant source used, they contain different proportions of the three basic monomers (p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol).
Development of a method of depolymerization by cleavage of the ether bonds will therefore contribute significantly to the upgrading of lignin. Now, direct depolymerization of lignin is difficult, as its structure is highly functionalized and branched and its steric hindrance may limit access of the catalyst to the active sites. Moreover, the chemical heterogeneity of lignin, which is due to the presence of several residues G, H, S present at variable levels depending on the nature of the plant, and to the presence of various types of C—O and C—C bonds of the aryl ether, aryl glycerol and β-aryl ether type, complicates the production of pure chemicals during transformation of lignin.
In view of the difficulty of carrying out direct depolymerization of lignin, scientists have synthesized chemically pure models that are representative of the ether bonds present in lignin, for studying the reactivity (J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius and B. M. Weckhuysen, Chem Rev., 2010, 110, page 3552). Most studies aiming at depolymerization of lignin have focused on these models and have not considered the complex structure of natural lignins. Examples of cleavage of C—O bonds of the β-O-4 unit on models of lignin using redox or reductive catalysis are given hereunder.                Bergman, Ellman et al. (J. M. Nichols, L. M. Bishop, R. G. Bergman, J. A. Ellman, J. Am. Chem. Soc. 2010, 132, pages 12554-12555) have developed a reaction of ruthenium-catalyzed redox cleavage of the C—O bond. The models of the β-O-4 units of lignin were cleaved with yields isolated ranging from 62 to 98%. The reaction takes place according to a tandem mechanism of dehydrogenation of the α-alcohol followed by reductive cleavage of the aryl ether.                    In addition, James et al. (A. Wu, B. O. Patrick, E. Chung and B. R. James, Dalton Trans., 2012, 41, page 11093) have shown that a ruthenium complex is able to catalyze the direct hydrogenolysis of the ketone equivalent of the β-O-4 unit with gaseous hydrogen. However, the authors observed that models of the β-O-4 unit containing the γ-OH function were not reactive.            Recently, Leitner et al. (T. vom Stein, T. Weigand, C. Merkens, Jurgen Klankermayer, W. Leitner, ChemCatChem, 2013, 5, pages 439-441) described a reaction of redox cleavage of C—O bonds of the β-O-4 unit, by intramolecular hydrogen transfer. This reaction employs a catalyst based on ruthenium (an expensive noble metal), a triphos ligand (a ligand that is also very expensive) and high temperatures (heating at 135° C.). Moreover, depending on the model of the β-O-4 unit used, the reaction may prove more or less simple to carry out.                        A vanadium catalyst was used by Toste et al. (S. Son and F. D. Toste, Angew. Chem. Int. Ed. 2010, 49, pages 3791-3794) for cleavage of C—O bonds of the β-O-4 unit and formation of aryl enones. This redox transformation is carried out in ethyl acetate at 80° C. The catalyst charge is 10 mol %, and after 24 hours the reaction may reach 95% conversion of the starting lignin model to aryl enone. As it is a redox reaction, the products obtained are generally highly oxygenated and therefore poor in energy.                    More recently, the same group (J. M. W. Chan, S. Bauer, H. Sorek, S. Sreekumar, K. Wang, F. D. Toste, ACS Catal., 2013, 3, pages 1369-1377) demonstrated the applicability of this method of redox cleavage to the degradation of lignin extracted from Miscanthus giganteus (elephant grass). The results of GC and 2D NMR studies of the degradation of dioxasolv and acetosolv lignin were similar to the data obtained with the lignin models, which confirms the selectivity of the method for the β-O-4 bonds. Moreover, only Miscanthus giganteus, which is a grass, was tested, not wood. Finally, using GC/MS, the authors were able to identify and quantify volatile phenolic compounds (such as vanillin, vanillic acid, syringic acid and syringaldehyde) produced in the reaction. However, no pure chemical could be isolated by this method, and partially characterized mixtures were obtained.                        In 2011, a selective method of hydrogenolysis of the aromatic C—O bonds in alkaryl ethers and diaryl ethers was developed by Sergeev and Hartwig (A. G. Sergeev and J. F. Hartwig, Science, 2011, 332, page 439). This method allows selective formation of arenes and alcohols starting from lignin models and using a soluble nickel-carbene complex. The reaction is carried out in m-xylene, under 1 bar of hydrogen and at temperatures ranging from 80 to 120° C. Use of this method allows cleavage of models of the 4-O-5 bond (diaryl ether), to give anisole, benzene, and phenols at moderate yields. Moreover, hydrogenolysis of models of the α-O-4 unit of lignin at 80° C. under 1 bar of hydrogen gives 3,4-dimethoxytoluene and 2-methoxyphenol at almost quantitative yields. Cleavage of the β-O-4 model in basic conditions is carried out without the presence of catalyst and supplies guaiacol at 89% yield but mixed with many other products.        The Toste, Ellman and Hartwig groups have combined their results on the reduction of lignin and of its models in homogeneous catalysis in international application WO2011003029. The precursors used are derivatives of vanadium, ruthenium and rhodium. Only the complexes based on vanadium and ruthenium were used for redox depolymerization of lignin extracted from Miscanthus giganteus. However, it was not possible to isolate or identify a pure chemical by this method, and partially characterized mixtures were obtained.        In 2009, Ragauskas et al. (M. Nagy, K. David, G. J. P. Britovsek and A. J. Ragauskas, Holzforschung, 2009, 63, page 513) succeeded in depolymerizing ethanol organosolv lignin (EOL) (ethanol-soluble) from pine in reducing conditions. In this study, classical heterogeneous catalysts as well as new homogeneous catalysts were used for cleaving diaryl ether and dialkyl ether bonds. Using the hydrogenolysis conditions: 5 MPa H2; 175° C.; 20 hours, the ruthenium catalyst is effective in increasing the solubility of lignin (solubility up to 96%) and contributes to its degradation. A decrease of the order of 10% to 20% in the weight-average molecular weight (Mw) was obtained (Mw=1900-2100 g/mol), which corresponds to a degree of polymerization (DP) of 10 to 11 monomer units (L. B. Davin, N. G. Lewis, Curr. Opin. Biotechnol., 2005, 16, pages 407-415). Moreover, according to the authors, hydrogenolysis of the diaryl ether and alkaryl ether groups is accompanied by a simultaneous hydrogenation reaction of the aromatic ring. Finally, identification as well as the detailed formation of the reaction products and cleavage pathways were not elucidated.        In 2013, the organocatalytic reduction of lignin model compounds was first described by Feghali and Cantat (E. Feghali, T. Cantat, Chem. Commun., 2014, 50, pages 862-865). They showed that B(C6F5)3 is an efficient, selective hydrosilylation catalyst for reductive cleavage of alkaryl ether bonds and particularly models of α-O-4 and β-O-4 units. Moreover, reduction takes place in mild conditions (room temperature, from 2 to 16 hours), and may be carried out with a source of hydride that is stable in air and inexpensive, such as polymethylhydrosiloxane (PMHS) and tetramethyldisilazane (TMDS). However, this method could not be extrapolated to the direct depolymerization of lignin.        
In view of the complex, heterogeneous and strongly hindered polymeric structure of lignin, which complicates its depolymerization, the methods of depolymerization developed in the literature and described above are generally carried out in harsh conditions of temperature and pressure and employ metals in larger catalytic amounts. Moreover, these methods were developed on chemically pure models, and few could be extrapolated to the reduction of lignin. In fact, only the methods of Ragauskas et al. (M. Nagy, K. David, G. J. P. Britovsek and A. J. Ragauskas, Holzforschung, 2009, 63, page 513) and of Toste et al. (S. Son and F. D. Toste, Angew. Chem. Int. Ed. 2010, 49, pages 3791-3794) could be extrapolated to lignin. The other methods did not work with lignin. The presence of several impurities, notably water, oxygen (O2), sulfur-containing molecules, phosphorus-containing molecules and sugar residues may deactivate the catalyst. These impurities may, for example, be derived from lignocellulose or from the method of extraction of lignin from lignocellulose.
There is therefore a real need for a method for depolymerizing lignin that overcomes the drawbacks of the prior art.
In particular, there is a real need for a method for depolymerizing lignin, said method:    being very efficient, reflected in a high level of conversion of lignin to smaller molecules containing 1 or 2 aromatic rings, and highly selective for certain bonds in lignin;    allowing aromatic molecules of high added value to be generated, in particular molecules containing 1 or 2 aromatic rings;    being simple to carry out;    and can be carried out in mild, industrially interesting operating conditions.